The Facts About Septic Tanks, and Other Threats to the Cape's Ground -Water Quality:

Another cause of ground-water contamination on the Upper Cape is effluent, or outflow, from septic tanks and cesspools. A majority of the homes on the Cape have septic systems.

During the operation of a septic system, domestic sewage is flushed into a large underground tank. The solids settle to the bottom of the tank and accumulated septage is pumped out every 2 years or so. The effluent flows out of the tank and into a series of underground trenches filled with gravel. The effluent trickles through the gravel into the soil where the organic matter decomposes.

For purification to work, however, effluent must move slowly through aerated soil or rock. That way, organisms can feed on the sewage and make it harmless before it moves very far. If the polluted water moves through the soil or rock too quickly, the organisms cannot decompose it, and the polluted water can contaminate the aquifer underneath. Geologic conditions on the Cape - the sandy, permeable soil and unconfined aquifer - make the ground water highly susceptible to septic-tank effluent contamination. The Cape's sand filters bacterial contamination well, but it allows other household toxins, such as paint thinner, to move straight through to the aquifer from which the drinking water supply is drawn.

In addition to septic tank systems, other sources of ground-water contamination in Cape Cod include wastewater treatment facilities; landfills; underground tank storage of fuel oil and home-heating oil in addition to gasoline; pesticide, herbicide, and fertilizer application in agricultural areas as well as salt application in residential areas; deicing salt-storage areas and salt application on highways; waste from industrial parks, leaking sewer lines, and so forth. Information on additional sources of ground-water contamination in Cape Cod is described by Frimpton and Horsley (1993).

A septic tank holds solid wast and releases wastewater into a leaching field that contains stone-filled trenches. On Cape Cod, septic tank wastewater frequently percolates to the aquifer below.